This invention relates generally to digital telephone exchanges and more particularly to a self diagnostic test feature of a digital telephone exchange which enables complete testing of a single time multiplexed channel without taking an entire trunk of time multiplexed channels out of service. This invention is related to the invention described in U.S. Patent Application No. 161,883, "Digital Telephone Switch with Simultaneous Dual PCM Format Compatibility" filed on behalf of Finley et al. on the same date as the present invention.
One of the advantages of digital telephone equipment is that it lends itself to testing by digital methods, which if properly implemented can eliminate some of the ambiguities attendant in the testing and operation of analog systems such as analog level drift. For the digital methods, bit patterns injected at various points should yield exactly predictable bit patterns at any point in the transmission path.
Many digital exchanges interface their switching subsystem (called herein the switching network) through which calls are routed, all or in part, to the outside world via Time-Division-Multiplexed (TDM) Pulse-Code-Modulated (PCM) four-wire circuits which can carry 24 or 30 or more four-wire speechpaths herein referred to as subchannels. The PCM circuits are herein referred to as "PCM groups". Each subchannel is allocated specific bit positions for speech or data and for channel associated line signaling for trunk use or subscriber line use or for unrelated use. Where the trunks employ common channel signaling, the line signaling bit positions may be used for that purpose, or even go unused, as might also be the case in certain data and audio applications.
A technique known as loopback is often used in telephone systems as a method of testing either the exchange equipment or the facilities that interconnect exchanges or the facilities between an exchange and the customer's premises. Many digital exchanges incorporate loopback, either entire PCM groups or a multiplex of PCM groups (called supergroups) in the digital domain, as a means by which the integrity of a connection or an entire exchange can be qualified, and by which defective componentry and subsections of an exchange can be isolated either automatically or manually.
Looping back all the incoming subchannels in the outgoing direction is generally called "incoming loopback". It is not widely used. Some exchanges may be able to perform this inbound loopback for an entire PCM group. Some PCM terminal equipment is capable of performing this function for test purposes.
Looping back the outbound speech and line signals of a subchannel so that it returns on the incoming input for that same subchannel is referred to herein as "outbound loopback". Another form of looping sometimes called loopback but referred herein as "turnaround" essentially cross-connects an outgoing trunk with a different incoming trunk so that an exchange can complete a test call to itself. One of the problems with outbound loopback and turnaround is that the distant exchange will often "see" spurious data and or line signals. The exchanges on the distant ends of different types of trunks prefer to see, under abnormal conditions or when temporarily busied out or when out of service for repair, different line signaling combinations, which when violated leads to confusion and possible congestion because that office may mark the trunks out of service at their end due to perceived problems.
In the case of PCM group outbound loopback, since the entire group is unusable under these conditions, it is most common to simultaneously transmit an alarm signal toward the distant end.
Flexibility in the treatment of the individual subchannels while not actually carrying traffic would allow more degrees of freedom in test capability commensurate with least disturbances to the distant office.